In port fuel injected engines operating with a three way catalytic converter, it is desired to maintain near stoichiometric conditions during most operating conditions. Such control is especially difficult during transient conditions where airflow is rapidly changing. Any significant deviations from stoichiometry, either rich or lean of stoichiometry, are outside the operating window of the three way catalytic converter and tend to degrade emission performance. Thus, various methods have been used to improve air-fuel ratio control.
A common approach to improve air-fuel control is to use a so called transient fuel algorithm as is known to those skilled in the art. In this approach, fueling delays and fuel storage (puddling) in the intake manifold are taken into consideration when controlling fuel injectors. In other words, these transient fuel algorithms account for fuel supplied from both fuel injectors and from the manifold fuel puddle to the cylinder.
Continuing with this approach, a desired cylinder fuel amount is found from measured or estimated cylinder air charge. Then, the desired cylinder fuel amount is used to calculate a required fuel injection amount, taking manifold fuel puddling into consideration.
Finally, the required fuel injection amount is used to calculate an injector pulse width, or opening duration. In this way, air-fuel ratio can be more accurately controlled to stoichiometric conditions. Such a system is described in U.S. Pat. No. 5,746,183.
It is also known to control airflow to provide a desired engine torque. In certain conditions, such as during tip-out conditions where a vehicle operator removes their foot from the accelerator pedal, it might be desirable to rapidly reduce engine airflow to rapidly reduce engine torque.
The inventors herein have recognized a disadvantage with known approaches for controlling air-fuel ratio with transient fuel algorithms during such rapid airflow transients. In particular, during these tip-out situations, transient fuel algorithms determine that fuel needs to be taken out of the intake ports since excessive fuel is being supplied by the manifold fuel puddle. However, such action is not possible since conventional fuel injectors can only inject fuel into the intake ports. Therefore, the best that the fuel injectors can perform is to inject no fuel. As a result, a rich excursion occurs and emission performance is degraded. In other words, even when no fuel is injected by fuel injectors, fuel in excess of the stoichiometric ratio is inducted from the manifold fuel puddle into the cylinder.